| Preservation potential of marine mollusks is believed to be mostly a result of shell mineralogy and depositional environment. However, little is known about how mineralogy and depositional environment interplay and affect preservation quality within a regional setting over a relatively short depositional period. To better understand preservation potential under these conditions, I examined mollusks from the Pierre Shale in the University Colorado Museum of Natural History invertebrate paleontology collection. This collection contains mostly ammonites and bivalves from 197 localities from the Western Interior Seaway that span an approximately 12 million year period of time. Specimens exhibit a range of preservation, from excellent (mostly unaltered original shell present) to very poor (only molds/casts present). Depositional settings include nearshore and offshore settings. Preservation quality of 107 cephalopods and 186 bivalves, from 58 localities were examined for the percent of original shell present. I examined the relationship between preservation quality, taxonomy, mineralogy, size, shape, and relative distance from shore. Although significant differences were found in the preservation quality of shells of differing taxonomy and mineralogies, the mineralogy itself is not likely to be a determinant of how well a shell is preserved. Rather, variables that are coincidental to mineralogy - like ecology (life habit) and post-mortem processes - affect shell preservation. Size and depositional environment are important for bivalve preservation quality, whereas cephalopod preservation does not appear to be associated with size, shape or depositional environment. |
